FIG. 8 shows a known heat pump as is described in “Technische Thermodynamik”, Theoretische Grundlagen und praktische Anwendungen, 14th revised edition, Hanser Verlag, 2005, pp. 278-279. The heat pump includes a closed cycle, within which a working substance, such as R 134a, circulates. Via a first heat exchanger 80 and the evaporator, so much heat is withdrawn from the soil, or the ground water, that the working substance evaporates. The working substance, which now is rich in energy, is extracted by the compressor via the suction line. Within the compressor 81, it is/it will be compressed, thus increasing pressure and temperature. This compression is performed by a piston compressor. The working substance, which has been compressed and exhibits a high temperature, now passes into the second heat exchanger 82, the liquefier. Within the liquefier, so much heat is withdrawn from the working substance by the heating or process-water cycle that the coolant, being subject to high pressure and high temperature, is liquefied. Within the choke or expansion member 83, the working substance is expanded, i.e. the working substance is relieved of stress. Here, pressure and temperature are reduced to such an extent that the working substance is again able to re-absorb energy from the soil or the ground water within the evaporator. Now the cycle is complete and starts again.
As can be seen from this, the working substance serves as an energy transporter so as to take up heat from the soil or ground water, and to give it off, within the liquefier, to the heating cycle. In this process management, the 2nd law of thermodynamics is complied with, said law stating that heat or energy only be transferred, “on its own”, can from a higher temperature level to a lower temperature level, and that inversely this may also occur by means of energy supply from outside, here by the driving work of the compressor.
FIG. 7 shows a typical h, log p diagram (h is the enthalpy, p the pressure of a material). An isobaric evaporation of the working substance takes place, between point 4 and point 1 in the diagram of FIG. 7, at low values for the pressure and the temperature (p1, T1). Here, the heat Q81 is supplied.
Ideally, a reversible compression of the working substance vapor to a pressure of p2 is performed, between point 1 and point 2, within an adiabatic compressor. The temperature rises to T2 in the process. A work of compression is to be supplied here.
Then, isobaric cooling of the working substance vapor from 2 to 2′ is performed at a high pressure p2. Overheating is reduced. Subsequently, the working substance is liquefied. Overall, the heat Q25 can be dissipated.
Within choke 83, the working substance is choked, in an adiabatic manner, from the high pressure p2 to the low pressure Pi. In the process, part of the liquid working substance evaporates, and the temperature falls to the evaporating temperature T1. In the h, log p diagram, the energies and characteristics of this process may be calculated by means of enthalpies, and may be illustrated, as is shown in FIG. 7.
The working fluid of the heat pump thus takes up, within the evaporator, heat from the surroundings, i.e. air, water, waste water or the soil. The liquefier serves as a heat exchanger for heating up a heating substance. Temperature T1 is slightly lower than the ambient temperature, temperature T2 is considerably higher and temperature T2′ slightly higher than the heating temperature necessary. The higher the temperature difference called for, the more work must be effected by the compressor. Therefore, it is desired to keep the rise in temperature as small as possible.
Thus, with regard to FIG. 7, a compression of the working material vapors is performed, in the ideal case, along the curve for the entropy s=constant up to point 2. From here up to point 3, the working material liquefies. The length of the distance 2-3 represents the useful heat Q. From point 3 to point 4, the working material is expanded, and from point 4 to point 1, it is evaporated, the distance 4-1 reflecting the heat withdrawn from the heat source. Unlike the T, s diagram, the magnitudes of the heat and of the work may be taken as distances in the h, log p diagram. Pressure losses within valves, within the pressure and suction lines, of the compressor, etc. change the ideal curve of the cyclic process in the h, log p diagram and reduce the effectiveness of the entire process.
With piston compressors, the working material vapor which has been sucked in initially has a lower temperature than the cylinder wall of the compressor, and thus absorbs heat from it. As the compression increases, the temperature of the working material vapor eventually increases to exceed that of the cylinder wall, so that the working material vapor gives off heat to the cylinder wall. Then, when the piston again sucks in and compresses vapor, the temperature of the piston wall is initially fallen below again and then exceeded, which leads to constant losses. In addition, overheating of the working material vapor which has been sucked in will be called for and necessary for the compressor to no longer suck in any liquid working material. What is also disadvantageous, in particular, is the heat exchange with the oil cycle of the piston compressor, which is indispensable for lubrication.
Any irreversible processes, such as heat losses during compression, pressure losses within the valves, and flow losses within the pressure line for liquefying and within the liquefier, will increase the entropy, i.e. the heat which cannot be retrieved. In addition, temperature T2, also exceeds the liquefying temperature. Such an “overheating enthalpy” is undesired, in particular because the high temperatures occurring in the process will accelerate the aging of the compressor and, in particular, of the lubricating oil within a piston compressor. Also, the effectiveness of the process is reduced.
The liquefied working material at a low temperature at the output of the liquefier would have to be expanded, within the context of an ideal cyclic process, via an engine, for example a turbine, so as to exploit the excess energy which was present in comparison with the state present at the temperature and the pressure prior to compressing. Because of the great expenditure necessary for this, this measure is dispensed with, and the pressure of the working material is abruptly reduced to the low pressure and the low temperature by the choke 83. The enthalpy of the working material remains approximately the same in the process. Due to the abrupt pressure reduction, the working material must partially evaporate to reduce its temperature. The evaporation heat necessary is derived from the working material exhibiting excess temperature, i.e. is not withdrawn from the heat source. The entirety of the losses caused by the expansion within choke 83 (FIG. 8) is referred to as expansion losses. These are energy losses because heat of a temperature T is converted to heat of a temperature T0. These losses may be reduced if the liquid working material can dissipate its heat to a medium having a temperature smaller than T. This undercooling enthalpy may be exploited by an internal heat exchange which, however, also necessitates additional expenditure in terms of equipment. Also in principle, the internal heat exchange has its limitation, because in the compression of the vapors, the overheating temperature T2 increases, whereby the gains achieved are partly cancelled out, and because also more thermal strain is put on the machine and the lubricating oil. Eventually, the overheating causes the volume of the vapor to increase, whereby the volumetric heat power decreases. This heat is utilized for preheating those vapors of the working material which flow to the compressor, only to the extent necessary for being sure that all droplets contained in the vapor of the working medium are converted to vapor.
In general, one may state that the ratio of the enthalpy difference between point 1 and point 4 and the enthalpy difference between point 2 and point 1 of the h, log p diagram is a measure of the economic efficiency of the heat pump process.
A working substance which is currently popular is R134a, the chemical formula of which is CF3—CH2F. It is a working substance which, even though it is no longer damaging to the ozone layer, nevertheless has an impact, in terms of the greenhouse effect, which is 1000 times higher than that of carbon dioxide. However, the working substance R134a is popular since it has a relatively large enthalpy difference of about 150 kJ/kg.
Even though this working substance is no longer an “ozone killer”, there are nevertheless considerable requirements placed upon the completeness of the heat pump cycle, to the effect that no molecules of the working substance will escape from this closed cycle, since they would cause considerable damage due to the greenhouse effect. This encapsulation leads to considerable additional cost when building a heat pump.
Also, one must assume that by the time the next stage of the Kyoto Protocol is implemented, R134a will be prohibited by the year 2015 because of the greenhouse effect, which has also happened to previous, considerably more damaging substances.
What is therefore disadvantageous about existing heat pumps, beside the fact of the harmful working substance, is also the fact that, due to the many losses within the heat pump cycle, the efficiency factor of the heat pump typically does not exceed a factor of 3. In other words, 2 times the energy that has been used for the compressor may be withdrawn from the heat source, such as the ground water or the soil. When considering heat pumps wherein the compressor is driven by electrical current, and when considering, at the same time, that the efficiency factor in current generation is perhaps 40%, one will find that—with regard to the overall energy balance—the use of a heat pump is very questionable. In relation to the source of primary energy, 120%=3·40% of heat energy are provided. A conventional heating system using a burner achieves efficiency factors of at least 90-95%, i.e. an improvement of only 25-30% is achieved at high technical and, therefore, financial expense.
Improved systems use primary energy for driving the compressor. Thus, gas or oil is burned to provide the compressor rating using the energy released by combustion. What is advantageous about this solution is the fact that the energy balance actually becomes more positive. The reason for this is that even though only about 30% of the source of primary energy may be used as driving energy, the waste heat of, in this case, about 70% can also be used for heating. The heating energy provided will then amount to 160%=3·30%+70% of the source of primary energy. What is disadvantageous about this solution, however, is that a household will nevertheless necessitate a combustion engine and a fuel store even though it has no longer a classical heating system. The expenditure made for engine and fuel storage must be added to the expense made for the heat pump, which, after all, is a highly closed cycle due to the coolant being harmful to the climate.
All of these things have resulted in that heat pumps have had only limited success in competition with other types of heating systems.